A SMALL, DOUBLE RING MICROSTRIP ANTENNA

Abstract: A small microstrip antenna (200) useful in portable electronic devices including pagers, telephones, and portable computers. The microstrip antenna (200) includes a ground plane layer (212), a dielectric layer (214), and a radiating patch (216) with a rectangular ring (250) formed therein. The radiating patch (216) is short circuited to the ground plane layer (212) along a shorted edge (222). The ring (250) may be positioned offset so that it is closer to the short circuited edge (222) than to the radiating edge (220). The antenna can fit within a standard PCMCIA slot in portable computers. The small size of the antenna allows orientation within a housing to optimize the radiation from the antenna. The antenna is useful in stand alone computers.

Description: A SMALL. DOUBLE RING MICROSTRIP ANTENNA Cross-reference is made to a commonly assigned copending patent application entitled "A Small Microstrip Antenna Having a Partial Short Circuit", Serial No. , filed the same date herewith. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to small microstrip antennas for portable electronic devices. Particularly, the invention relates to microstrip antennas for portable electronic devices that must operate in close proximity to humans or computer equipment and that are small enough to fit within a credit card size pager, a portable telephone, or a portable computer. Description of Related Art Small antennas are used in portable electronic devices such as pagers and portable telephones to receive radio frequency (rf) signals. For those uses, the small antennas have been positioned internally within their structure. However, for portable computer uses, conventional technology requires an external antenna in order to receive or transmit a signal. It would be an advantage to provide a small antenna that can be positioned internally inside a portable computer. One conventional small antenna suitable for pagers and portable telephones is a microstrip antenna. A typical two-conductor microstrip antenna includes a layered planar structure having a conductive ground plane, a conductive radiating patch, and a dielectric layer positioned between the radiating patch and the ground plane. The ground plane of the antenna acts as a type of shield against adjacent materials such as circuit components and other metallic materials. Microstrip antennas have been developed in half-wavelength structures and quarter-wavelength structures. The length of a conventional half-wavelength microstrip antenna is about one-half of the wavelength inside the dielectric. In a quarter-wavelength antenna, the length is reduced in half by short circuiting across one of the radiating edges to short the radiating patch to the ground plane. In a half -wavelength antenna, the ground plane and the radiating patch are unconnected, open circuits along both radiating edges. In a quarter-wavelength antenna, one of the radiating edges is short circuited along its entire length. Even though the size of the quarter wavelength antenna is substantially reduced below that of the half-wavelength antenna, it would be an advantage to reduce size still further if the antenna's efficiency can be retained within reasonable limits. One advantage of a smaller antenna is an ability to fit within small areas. If the antenna's requirements specify a fixed weight or a limited volume, the effect of reducing surface area is to allow an increased dielectric thickness, which substantially increases efficiency. One problem with small antennas has been the substantial gain reduction that, previously, had been associated with decreasing size. Attempts have been made to design small microstrip antennas with high gain. For example, it is known that a small slit formed in the radiating patch changes the reactance, which can substantially reduce antenna size at the expense of gain. For the same reason, a "C" shaped opening can also be used to reduce antenna size at the cost of a substantial reduction in gain. Furthermore, it is known that the quarter wavelength antenna is more efficient if the dielectric is extended from the one radiating edge. However, the dielectric extension disadvantageously increases the length and consequently increases the overall size and weight of the device. It would be an advantage to provide a microstrip antenna that provides a high level of efficiency per volume. Such a microstrip antenna could reduce costs of portable electronic equipment while mamtaining high efficiency within a small volume. Every microstrip antenna emits radiation in a particular pattern determined by its shape, which is often termed the "geometry" of the antenna. Many different geometries for microstrip antennas have been developed. For a particular use, an antenna designer will choose the geometry that most closely matches the particular needs of that use. A properly selected geometry allows efficient transmission and reception of rf signals. One common microstrip antenna geometry is a solid rectangle or square. Many geometries have been described and catalogued. For example, numerous geometries are shown in the Handbook of Microstrip Antennas. James and Hall, eds., Peter Peregrinus Ltd., London, UK, 1989, pp. 24-39. For portable electronic applications, the radiation pattern and the resonance frequency can be greatly affected by adjacent physical objects such as electrical circuits, computer equipment, and people. The shift in resonance frequency can be severe. For example, the frequencies may shift by tens of megahertz or more, which can render the microstrip antenna effectively useless. Operating characteristics such as the gain of the antenna can also be affected by adjacent physical objects. It would be an advantage to provide a microstrip antenna having a geometry that exhibits a reduced body effect. Such an antenna would be useful in a number of uses of portable electronic equipment such as pagers that must operate in the presence of people. It is believed that many conventional microstrip geometries are disadvantageous around certain physical objects because conventional microstrip geometries are excited mainly by electric current rather than magnetic current. Generally, it is known that if a microstrip antenna can be excited mainly by a magnetic current rather than an electric current, the effect of adjacent materials, including human bodies on an antenna performance can be considerably reduced. Furthermore, in such a microstrip antenna, the image of the magnetic current with respect to the ground plane, or any other adjacent metallic material, or human body, would provide enhanced radiation in front of the microstrip antenna. Another disadvantage of conventional microstrip geometries is the nonisotropic radiation pattern, (i.e., the pattern is not evenly distributed) with a peak directed perpendicular to the microstrip plane. If the antenna is located within a housing for electrical equipment, the positioning of the peak can be highly critical for proper transmission and reception of signals. An "isotropic" antenna is one whose radiation pattern is approximately evenly distributed in all directions, i.e., approximately omnidirectional. It would be an advantage to provide an isotropic microstrip antenna that radiates approximately omnidirectionally without a radiation peak directed towards the electronic device to which it is connected or to any people located close by. Furthermore, it would be an advantage to provide a microstrip antenna having an approximately square shape that can be installed in an approximately square space. Such a shape would allow a manufacturer to position the microstrip antenna in a small area, and to produce a radiation pattern optimized for its relationship with adjacent circuits, components, and bodies. SUMMARY OF THE INVENTION The present invention provides a microstrip antenna with a small area without substantial reduction in gain. The microstrip antenna provides increased efficiency per antenna volume, thereby reducing size and cost. The smallness and relatively high gain of the microstrip antenna is useful in portable electric devices including pagers and telephones. Furthermore, the antenna is small enough to fit within a standard PCMCIA slot, thereby opening uses in portable computers that have a PCMCIA card. Because of the small size of the antenna, its location and orientation inside the computer can be easily optimized to get the maximum radiation power from the antenna inside the computer. The present invention also provides a microstrip antenna that has an approximately isotropic radiation pattern, a minimum human body effect, and a -minimum computer effect. The microstrip antenna experiences only -minimum pattern degradation inside a PCMCIA slot. For example, the present invention -minimizes frequency shifting effects in the presence of objects proximate thereto. Furthermore, the antenna structure provides band-limiting filter properties inherent in microstrip antennas which minimize the reception of unwanted signals. Moreover, the geometry of the antenna is designed such that the radiation pattern is more omnidirectional than conventional microstrip antennas. The microstrip antenna provides improved performance when positioned adjacent to a human body as well as inside a computer. The double ring microstrip antenna includes a ground plane layer comprising a conductive material, a radiating patch layer comprising a conductive material, and a dielectric layer positioned between the radiating patch layer and the ground plane layer. In the double ring geometry, the microstrip antenna is preferably short circuited along one of its edges. The short circuit could be a full short circuit along the entire length of an edge, or a partial short along only a portion of the edge. The addition of the short circuit creates a "mirror" image of the radiating patch in the ground plane layer. A rectangular ring is formed in the radiating patch. Preferably, the ring is offset so that it is closer to the shorted edge than to the radiating edge; however, in some embodiments the ring could be centered, or shifted closer to the radiating edge. Because a "mirror" image of the radiating patch is created in the ground plane layer, a double ring effect is created in the antenna. The distance between the radiating edge and a ring edge proximate thereto is maximized in order to reduce the length of the antenna. The desired input impedance can be selected by varying the length between the edge of the ring and the shorted side, and varying the two dimensions between the edges of the rings and the two nonradiating edges. Additional features, including cutting the edges of the nonradiating edges closely to the dielectric, enhance the isotropic radiation pattern. It is believed that the ring geometry improves antenna performance in the presence of humans and computers because it is excited mainly by magnetic currents instead of electric currents. This magnetic current excitation, combined with the shape and dimensions of the antenna, enable the described antenna to be only minimally affected by adjacent circuits, and to provide relatively high gain, a uniform radiation pattern, and a minimum human body effect. The antenna can provide these advantages even when operating in a PCMCIA slot. A partial short circuit along the shorted edge can be combined with a ring geometry to provide a double offset ring partially shorted microstrip antenna. The overall result is an antenna structure that combines the advantages of both of the structures. For pagers, the small size double rectangular ring microstrip antenna with a partial short has been designed to resonate at 931.5 MHz. Tests suggest that the average gain achieved by this antenna structure ranges from 0.0 dB to -5.0 dB with respect to an isotropic antenna. Data taken on this antenna suggest that, unlike conventional microstrip antennas, a double ring partially shorted microstrip antenna constructed in accordance with the present invention shows no appreciable frequency shift when operated inside of a PCMCIA slot or when in proximity to a human body. The features and advantages described in the specification are not all inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purpose, and therefore resort to the claims is necessary to determine the inventive subject matter. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a partially shorted microstrip antenna. Fig. 2 is a perspective view of a double rectangular ring microstrip antenna. Fig. 3 is a perspective view of a partially shorted rectangular ring microstrip antenna. Figs. 4A, 4B, and 4C are perspective views of a microstrip antenna in various orientations within a pager. Fig. 5 is a perspective view of a microstrip antenna in a PCMCIA card to be inserted in a portable computer or pager. Fig. 6 is a perspective view of a portable telephone with a cutaway section showing a microstrip antenna positioned therein. Fig. 7 is a perspective view of a computer housing with a slot for receiving a microstrip antenna. Fig. 8 is a block diagram of a microstrip antenna and associated circuits in a PCMCIA card. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figures 1 through 8 of the drawings disclose various embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention. The following description describes a microstrip antenna that has been designed with dimensions suitable for application in pagers, which operate at 931.5 MHz. Therefore, the dimensions given for the preferred embodiment correspond to that specific frequency of operation. It will be apparent to one skilled in the art that a microstrip antenna for other frequencies of interest can be designed by varying the antenna dimensions. Reference is now made to Fig. 1 which shows a partially short circuited microstrip antenna 100. A ground plane comprising a conductive material is formed in the ground plane layer 112. A dielectric layer 114 is affixed to the ground plane layer 112. A radiating patch 116 is formed on the side of the dielectric layer 114 opposite the ground plane layer 112. The dielectric layer 114 preferably has a very low tangent loss and a controlled dielectric constant of 2.94 ± 0.04. The radiating patch layer preferably has a thickness of 1.0 oz./m 2 of copper foil. The partially shorted microstrip antenna 100 includes four sides, including a radiating edge 120 and opposite thereto a partially shorted edge 122. The other two edges include a first side edge 124 and opposite thereto a second side edge 126. Even though the side edges 124 and 126 are often termed "nonradiating" edges elsewhere, an amount of radiation will normally be emitted from them. However, that amount is small in comparison to the radiation emitted from the radiating edge 120. The radiating edge 120 is open circuited along its entire length, as is the first side edge 124 and the second side edge 126. The partially shorted edge 122 is short circuited along a portion of its length. Specifically, the partially shorted edge 122 includes a shorted section 130 that couples the radiating patch layer 116 with the ground plane layer 112. The shorted section 130 comprises any conductive material, preferably a copper foil such as the foil that comprises the radiating patch layer 116. The shorted section 130 has a width Z s which spans less than the entire width Z w of the partially shorted edge 122. A first open circuited section 132 having a length Z r si is positioned between the shorted section 130 and the first side edge 124. A second open circuited section 134 having a length / rS2 is positioned between the shorted section 130 and the second side edge 126. As illustrated, the shorted section 130 is formed in one continuous section. However, in other embodiments, the shorted section 130 may comprise two or more sections (not shown) that are separated. It is believed that the total length of all shorted sections, whether connected directly or not, must be sufficient to provide a mirror image of the radiating patch in the ground plane. Therefore, the total length / s should not be reduced below that length which provides such an adequate mirror image. In other words, the length / s of the short must be sufficient to provide an image in the ground plane layer 112. The width / s of the shorted section 130 is chosen primarily to satisfy the required input impedance of 50 ohms. It has been observed that changing the width / s effects the input impedance, and therefore varying the width s as a percentage of the total width w can be useful in tuning the antenna. However, it has been experimentally observed that if the width Z s is decreased below, for example 10%, then the antenna does not perform efficiently, or it may not work at all. Of course, if the length / s of the short is reduced to zero, the antenna's properties will shift to those of a half-wavelength antenna. On the other hand, if the length l s is increased above, for example 90%, then the partially shorted microstrip antenna 100 will begin to assume the properties of a conventional quarter-wavelength antenna. Therefore, the length / s should be between 10% and 90% of the value of / w . It is currently preferred that the length / s of the short circuit is within the range of 20% to 50% of the length / w of the entire partially short circuited edge 122. The currently preferred length / s is approximately 30%. In a typical quarter-wavelength antenna (not shown) the length between the fully short circuited edge and the radiating edge is approximately equal to one quarter of the wavelength of the resonance frequency in the dielectric material. However, in the described embodiment, the length of the antenna from the partially shorted edge 122 to the fully radiating edge 120, the length / a , is shorter than the quarter-wavelength for a given resonance frequency. It is believed that the partial short circuit provided by the shorting section 130 reduces the resonance frequency, which allows the length Z a of the antenna 100 to be smaller than a quarter-wavelength for a given resonance frequency. In the preferred embodiment, for a resonance frequency of 931.5 MHz, the length l a of the antenna is 30 mm, which is approximately 40% shorter than any known conventional microstrip antenna with an equivalent gain. The width / s of the shorting section 130 is approximately 9 mm which is approximately 30% of the entire width. The length l IS _ of the first open circuit section 132 is approximately 10.5 mm, and the length rS2 of the second open circuit section 134 is approximately 10.5 mm. In the preferred embodiment, the width / w is made approximately equivalent to the length Z a , (30 mm) so that an approximately square structure is provided. If the width / w were to be made larger, gain would be increased somewhat. However, it was found that the width / w could be reduced to approximately that of the length l a without a large reduction in gain. The square structure provides advantages including convenience in installation and a reduced size. In other embodiments, the width Z w could be made wider for more gain, but this would naturally increase the overall size of the microstrip antenna 100. Also, the width Z w could be made narrower to decrease the overall size of the microstrip antenna 100, but this would reduce gain. In the preferred embodiment, the first side edge 124 and the second side edge 126 are cut closely so that the respective edges of the ground plane layer 112, the dielectric layer 114 and the radiating patch layer 116 are approximately flush (i.e., evenly lined up). It is believed that cutting the edges 124 and 126 evenly provides a more isotropic radiation pattern. A feed point 140 is positioned proximate to the shorted section 130, approximately centralized within its width / s . The position of the feed point 140 is selected so that the input impedance is 50 ohms. The feed point 140 is connected to a conventional coaxial cable 142. The coaxial cable 142 is coupled so that its outer conductor is coupled to the ground plane layer 112, and its center conductor is coupled to the radiating patch layer 116 at the feed point 140. However, other conventional techniques for connecting a transmission line to a microstrip antenna could be utilized. Because the characteristic impedance of the coaxial transmission line 142 and the receiver is typically 50 ohms, in the preferred embodiment the input impedance of the antenna 100 is selected to match 50 ohms. A number of antenna characteristics apparent to one skilled in the art, including the feed point 140 and others that will be discussed, affect the input impedance, all of which should be taken into account in designing the input impedance. A number of optimizations have been included in the microstrip antenna 100, in order to reduce size substantially with little or no corresponding reduction in gain. One advantage of size reduction is that the width W& of the dielectric layer can be made wide to further increase the gain at a small cost. In the preferred embodiment, the width w& is between 0.015 inch and 0.090 inch, preferably approximately 0.060 inch. Of course, the area of the antenna is not changed by an increase in the width W d of the dielectric layer 114. Furthermore, an exposed dielectric section 150 is provided to increase gain. The radiating patch 116 does not extend over this section 150, leaving the dielectric "exposed". The ground plane layer 112 continues until the edge 120. Reference is now made to Fig. 2 which is a perspective view of a microstrip antenna 200 including a rectangular ring 250 to be described. As illustrated in Fig. 2, a ground plane 212 comprising a conductive material is formed in a ground plane layer 212. A dielectric layer 214 is affixed to the ground plane layer 212. On the side of the dielectric layer 214 opposite the ground plane layer 212, a radiating patch 216 is formed. The dielectric layer 214 preferably has a very low tangent loss and a controlled dielectric constant of 2.94 ± 0.04. Preferably, the radiating patch layer has a thickness of 1.0 oz./m 2 of copper foil. The microstrip antenna 200 includes four sides, including a radiating edge 220 and opposite thereto a shorted edge 222. The other two edges include a first side edge 224 and a second side edge 226. The first radiating edge 220 is open circuited along its entire length, as is the first side edge 224 and the second side edge 226. As illustrated in Fig. 2 and noted above, the shorted edge 222 is fully shorted along its entire length. However, as will be explained particularly with reference later to Fig. 3, in other embodiments the shorted edge 222 may comprise a partial short along its length. A feed point 240 is positioned proximate to the shorted edge 222, and approximately centralized over a rectangular ring 250. The ring 250 has a rectangular shape with a width / ww and a height wh - The width l r of the microstrip antenna 200 is, of course, greater than the width of the ring / ww - Preferably, the width l τ is approximately 30 mm, and the width / ww is approximately 18 mm. The ring 250 is positioned apart from the first side edge 224 by a length / ws ι, and the second side edge 226 is positioned apart from the ring 250 by a length WS 2. Preferably, the lengths / ws ι and / WS2 are equal. Also preferably the lengths / WS/ and Z WS2 are 6 mm, however they can be varied to change the input impedance. The length l m of the microstrip antenna 200 is of course greater than the height wh of the ring 250. Preferably, the length l m is 30 mm and the height / Wh is 9 mm. The ring 250 is positioned apart from the radiating edge 220 by a length / g , and the ring 250 is positioned apart from the fully shorted edge 222 by a distance l ws . The ring 250 may be offset towards the shorted edge 222 to reduce the total length of the antenna; in other words, the length / g may be greater than the length Z ws . It is believed that a larger length / g reduces the length / nr of the microstrip antenna. It is presently preferred that the length Z g be about one-half of the total length Z nr of the ring antenna 200, which in the preferred embodiment is about 15 mm. Also preferably, the length Z ws ι, / WS2 , and / ws are 6 mm, however any of these lengths can be varied to change the input impedance. The rectangular shape of the ring 250 may be square. However, in other embodiments the ring 250 could have any shape, and it may be positioned anywhere within the radiating patch 216. The ring 250 could be square, for example, or it could be offset towards the radiating edge 220, the side edge 224, or the second side edge 226. Of course, varying the shape and position of the ring will also affect other antenna operating characteristics, including the input impedance. Preferably, an exposed dielectric section 270 is provided to increase gain. The radiating patch 216 does not extend over the exposed dielectric section 270, leaving the dielectric "exposed". The ground plane layer 212 continues until the edge 220. Conventional techniques teach that an exposed dielectric portion will increase gain, however in the preferred embodiment the length a of the exposed dielectric section 270 is substantially smaller than conventional teaching would suggest. It is believed that the double ring geometry of the antenna 200 allows the length I d to be made smaller without substantial reduction in gain. In the preferred embodiment, the length d is in the range between 2.0 and 3.0 mm. Conventional references suggest that this width should be substantially larger, for example 35% of the total height / nr which, if followed, would require the exposed dielectric section 270 to be about 15 mm (more than five times larger) in the preferred embodiment. Reference is now made to Fig. 3 which is an illustration of a partially short circuited, dual rectangular ring microstrip antenna 300. The antenna 300 combines the partial short circuit feature of the microstrip antenna 100 discussed with reference to Fig. 1 with the rectangular ring 250 discussed with reference to the microstrip antenna 200 of Fig. 2. Each of these features has already been fully discussed with reference to the above figures, and therefore reference is made to those figures for more details of the microstrip antenna 300 of Fig. 3. The following is a brief description of the structures illustrated in Fig. 3. A ground plane layer 312 comprising a conductive material is formed with a dielectric layer 314 and a radiating patch 316 is formed on the opposite side of the dielectric layer 314. The microstrip antenna 300 has an approximately square shape including four sides each being approximately 30 mm. The antenna 300 includes a radiating edge 320 and opposite thereto a partially shorted edge 322. The other two edges include a first side edge 324 and a second side edge 326 opposite thereto. The radiating edge 320 is open circuited along its entire length, as is the first side edge 324 and the second side edge 326. The partially shorted edge 322 is short circuited along a portion of its length Z r . Specifically, the partially radiating edge 322 includes a short circuited section 330 that couples the radiating patch layer 316 with the ground plane layer 312. The short circuited section 330 comprises any conductive material. The short circuited section 330 has a width / s which spans the length less than the entire width l r of the partially radiating edge 322. For example, the width can be in the range of 20% to 50% of the width l τ , however preferably the length / s is approximately equal to 30% (9 mm) The width Z s of the short circuited section 330 can be selected experimentally so that a required input impedance can be obtained at a selected resonance frequency. A first open circuited section 332 having a length / rs ι is positioned between the short circuited section 330 and the first side edge 324. A second open circuited section 334 having a length l→tsi is positioned between the short circuited section 130 and the second side edge 326. Preferably, the lengths / rs ι and / rS2 are equal, and approximately 10.5 mm. A feed point 340 is positioned proximate to the short circuited section 330, approximately centralized within its width Is. The position of the feed point 340 is selected so that the input impedance is 50 ohms. The feed point 340 is connected to a conventional coaxial cable 342, which is coupled so that its outer conductor is coupled to the ground plane layer 312 and the center conductor is coupled to the radiating patch layer 316 at the feed point 340. An exposed dielectric section 348 is provided to increase gain of the antenna 300. The radiating patch 316 does not extend over the section 348, leaving the dielectric "exposed". The ground plane layer 312 continues until the edge 320. The microstrip antenna 300 also includes a rectangular ring 350. A feed point 340 is approximately centralized over the rectangular ring 350. The rectangular ring 350 has a rectangular shape with a length / ww and a height /wh. Preferably, / ww is 18 mm and / wh is 9 mm. The ring 350 is positioned apart from the first side edge 324 by a length / ws i / and from the second side edge 326 by a length / WS 2- Preferably, the lengths Z ws ι and / WS2 are equal and 6 mm. The rectangular ring 350 is positioned apart from the radiating edge 320 by a length / g which is preferably 15 mm, and the ring 350 is positioned apart from the shorted edge 322 by a distance / ws which is preferably 6 mm. The ring 350 is preferably offset towards the shorted edge 322; in other words, the length / g is greater than the length / ws - The length / g is chosen to provide a reduction in size, and is preferably approximately equal to one-half of the total length, / nr , of the microstrip antenna 300. Also preferably, the length / ws between the rectangular ring 350 and the shorted edge 322 is approximately equal to the length / ws ι and Z W s2- The dimensions for the preferred embodiment assume a resonance frequency of 931.5 MHz for pager applications. It should be apparent to one skilled in the art that the dimensions can be varied to change antenna characteristics including resonance frequency and impedance. Other optimizations have been included in the preferred embodiment. For example, the side edges 324 and 326 are cut flush with each other. It is believed that this provides a more isotropic radiation pattern. The radiation pattern had been experimentally observed to vary less than 5 dB both inside a computer environment and outside a computer environment as the antenna is turned through a 360 degree arc. Furthermore, the filtering aspects of the antenna 300 in the preferred embodiment have been found in the laboratory to give rejection in excess of 20 dB with respect to the in-band gain at frequencies greater than 10 MHz from the band center. Reference is now made to Figs. 4A, 4B, and 4C, which illustrate three different positions in which a microstrip antenna 400 can be situated within a pager 402. Fig. 4A illustrates a pager 402a having a microstrip antenna 400a with a first side edge 406 facing outward. Fig. 4B illustrates a pager 402b having an antenna 400b with a second side edge 410 positioned facing outward. Fig. 4C illustrates a third microstrip antenna 400c positioned with its radiating edge 420 facing outward. The presently preferred embodiment is illustrated in Figs. 4A and 4C, in which the side edge 406 or 410 faces outward from the edge of the pager. In another embodiment, (not shown) the partially shorted edge of the microstrip antenna could face outward from the pager. Either of these configurations will be operable, and each configuration has advantages that are highly dependent upon the particular circuit configuration of the pager 402 and other factors. Figs. 4A, 4B, and 4C also illustrate the flexibility that the small microstrip antenna 400 provides for installation within a electronic device such as the pager 402. Reference is now made to Fig. 5 which is an illustration of a small portable computer 500 having a PCMCIA slot 510 provided therein. The PCMCIA slot 510 is constructed in accordance with a well-known PCMCIA standard presently used within the computer industry, and has been previously used for inserting additional memory and other devices, such as programs, into small portable computers. A microstrip antenna 520 is illustrated positioned within a PCMCIA card 530. A plurality of connectors 540 are provided to connect to matching connectors (not shown) within the PCMCIA slot 510. The PCMCIA card 530 has standard dimensions of 85.60 mm for length Z p illustrated at 532, and 54.0 mm for a length / wp illustrated at 534. The thickness of the PCMCIA card varies dependent upon type: a type I card has a width of 3.3 mm, and a type II card has a thickness of 5.0 mm. The PCMCIA standard is incorporated by reference herein. Of course, the exact dimensions of the PCMCIA card are not essential to practicing the present invention, and other housings, with other dimensions could be utilized. However, the PCMCIA card 530 size is particularly useful because it has already attained the status of a well-known standard within the computer industry. Reference is briefly made to Fig. 8 which is a block diagram of the antenna 520 and related circuits in the PCMCIA card 530. The antenna 520 is coupled by any suitable means to a conventional radio frequency (rf) front-end 542 to receive signals from the antenna 520 and output them to a digital processing and interface circuit 546. Conventional circuits are included within the digital processing interface circuit 546 in order to receive the rf signals and interface through the output connectors 540 with any suitable electronic equipment such as a computer circuit. Reference is again made to Fig. 5. The PCMCIA slot 510 is usually separated from the remainder of the portable computer 500 by a metal case. Tests have indicated that an antenna 520 constructed in accordance with the present invention will radiate satisfactorily in all directions from a slot such as the PCMCIA slot 510 even if it is encased in metal. It has been observed that the preferred position within the slot is as indicated in Fig. 5, with one of the side edges, for example the side edge 550, facing outward from the slot 510. Reference is now made to Fig. 6 which is a perspective view of a portable telephone 600 showing a cut-away view of a partially short circuited, dual rectangular ring microstrip antenna 610 installed therein. The microstrip antenna 610 could be easily installed in the handset of a portable telephone, and could operate effectively therein. The antennas, such as the microstrip antennas 400 in Figs. 4A, 4B, and 4C, the antenna 520 in Fig. 5, and the antenna 610 in Fig. 6 can of course be sized according to the needs of the users. Particularly, the overall dimensions of the microstrip antenna, the dimensions of the shorting strip, and the dimensions of the rectangular ring can be adjusted to meet the desired resonance frequency and the needs of the user. For example, the antennas 520 and 610 which are used for portable computers and portable telephones respectively will have different dimensions from that described with reference to the preferred embodiment which is useful in pagers, although the overall proportions will preferably remain approximately the same. Reference is now made to Fig. 7. A housing 700 for a computer unit 704 is illustrated. The computer unit 704 can be any of a wide variety of computer units, including large portable units, desktop computers, and work stations, among others. A slot 710 is illustrated on a side 720 of the computer housing 700. An antenna case 730 that houses a microstrip antenna 740 is provided to fit within the slot 710 in the computer housing 700. It should be apparent to one skilled in the art that, in other embodiments, the antenna 740 could be permanently installed in the computer housing 700, without the necessity of the slot 710. However, it is useful that the antenna 740 be positioned in an opening 750 in the housing 700 in order to receive and transmit efficiently. From the above description, it will be apparent that the invention disclosed herein provides a novel and advantageous microstrip antenna. The foregoing discussion discloses and describes exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics, and thus, the described embodiment is not restrictive of the scope of the invention. For example, although Fig. 2 illustrates a single rectangular ring 250, it should be apparent to one skilled in the art that a mirror image of the rectangular ring 250 is formed in the ground plane layer 212 as a result of the shorted section 230. Therefore, a half-wavelength antenna could be constructed in accordance with the present invention, with two rings being formed symmetrically about a center line between the two radiating edges of a half-wavelength antenna. The following claims are indicative of the scope of the invention. All variations which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims: CLAIMS WHAT IS CLAIMED IS: 1. A double ring microstrip antenna comprising: a ground plane layer comprising a conductive material; a radiating patch layer comprising a conductive material; a dielectric layer positioned between the ground plane layer and the radiating patch layer; a shorting conductor coupling the radiating patch layer to the ground plane layer, said shorting conductor positioned on a shorted edge of the microstrip antenna; wherein said double ring microstrip antenna defines a radiating edge positioned opposite the shorted edge, a first side edge, and a second side edge; and wherein said radiating patch layer includes a rectangular ring formed therein, said rectangular ring having four sides including a first side proximate to the radiating edge, a second side parallel to the shorting conductor, a third side proximate to a first side edge, and a fourth side proximate to a second side edge. 2. The double ring microstrip antenna of Claim 1 wherein said rectangular ring is offset by said second edge being positioned substantially closer to the shorted edge than the first edge is positioned to the radiating edge. 3. The double ring microstrip antenna of Claim 2 wherein said offset rectangular ring is positioned closer to said shorted edge than to the fully radiating edge by a factor approximately between 1.25 and 2.0. 4. The double ring microstrip antenna of Claim 1 wherein said rectangular ring defines a width of Z ww , between the third side and the fourth side, and also defines a height / wh between the first side and the second side. 5. The double ring microstrip antenna of Claim 4 wherein said width / W w is approximately 1.5 times greater than the height /wh- 6. The double ring microstrip antenna as claimed in Claim 1 wherein a length / g is defined between the first side and the radiating edge, a length / ws is defined between the second side and the shorting conductor edge, a length / ws ι is defined between the third edge and the first side edge, and a length / ws is defined between the fourth side and the second side edge, and wherein said length / ws ι is approximately equal to the length Z WS 2- 7. The double ring microstrip antenna as claimed in Claim 6, wherein said length / ws is approximately equal to Z ws ι and Zws2- 8. The double ring microstrip antenna as claimed in Claim 6, wherein said lengths Z ws ι, Z WS2 / and Z ws equal approximately one-fifth of a length Z nr defined between the radiating edge and the shorted edge. 9. The double ring microstrip antenna as claimed in Claim 6 wherein said length Z g equals approximately one-half of a length Z nr defined between the fully radiating edge and the partially shorted edge. 10. A double ring, partially shorted microstrip antenna comprising: a ground plane layer comprising a conductive material; a radiating patch layer comprising a conductive material; a dielectric layer positioned between the radiating patch layer and the ground plane layer; and a short circuited section on a partially shorted edge of the microstrip antenna, said short circuited section comprising a conductive material, said short circuited section coupling a length between ten percent and ninety percent of the total length of the partially shorted edge; wherein said double ring microstrip antenna defines a radiating edge positioned opposite the shorted edge, a first side edge, and a second side edge; and wherein said radiating patch layer includes a rectangular ring formed therein, said rectangular ring having four sides including a first side proximate to the radiating edge, a second side proximate to the partially shorted edge, a third side proximate to a first side edge, and a fourth side proximate to a second side edge. 11. The microstrip antenna of Claim 10 wherein the short circuited section comprises a length between twenty and fifty percent of the length of the short circuited edge. 12. The partially shorted microstrip antenna of Claim 10 wherein said partially shorted edge, said radiating edge, said first edge, and said second side edge have a length that is approximately equal, so that the shape of the microstrip antenna is approximately square. 13. A microstrip antenna for portable electronic equipment that includes a housing for said portable electronic equipment, comprising: a ground plane layer comprising a conductive material; a radiating patch layer comprising a conductive material; a dielectric layer positioned between the ground plane layer and the radiating patch layer; a shorting conductor coupling the radiating patch layer to the ground plane layer, said shorting conductor positioned on a shorted edge of the microstrip antenna; wherein said double ring microstrip antenna defines a radiating edge positioned opposite the shorted edge, a first side edge, and a second side edge; and wherein said radiating patch layer includes a rectangular ring formed therein, said rectangular ring having four sides including a first side proximate to the radiating edge, a second side parallel to the shorting conductor, a third side proximate to a first side edge, and a fourth side proximate to a second side edge. wherein said microstrip antenna is positioned within said housing for the portable electronic equipment. 14. The microstrip antenna of Claim 13 wherein said microstrip antenna is positioned within said housing so that the first side edge is positioned proximate to said housing and facing outward therefrom. 15. The microstrip antenna of Claim 13 wherein said portable electronic equipment includes a pager, and said microstrip antenna is positioned within said pager housing. 16. The microstrip antenna of Claim 15 wherein said pager comprises a rectangular shape having a length, a width, and a thickness, and a first end defined by a width and a thickness, and wherein said microstrip antenna includes a first side edge, a second side edge opposite thereto, and a radiating edge opposite the partially shorted edge, and wherein said microstrip antenna is positioned within said pager housing so that the first side edge is adjacent to the first end of the pager. 17. The microstrip antenna of Claim 13 wherein said portable electronic equipment includes a portable telephone. 18. A microstrip antenna assembly for inserting into a slot in a housing for electronic equipment, said microstrip antenna assembly comprising: a microstrip antenna including a ground plane layer comprising a conductive material; a radiating patch layer comprising a conductive material; a dielectric layer positioned between the ground plane layer and the radiating patch layer; a shorting conductor coupling the radiating patch layer to the ground plane layer, said shorting conductor positioned on a shorted edge of the microstrip antenna; wherein said double ring microstrip antenna defines a radiating edge positioned opposite the shorted edge, a first side edge, and a second side edge; and wherein said radiating patch layer includes a rectangular ring formed therein, said rectangular ring having four sides including a first side proximate to the radiating edge, a second side parallel to the shorting conductor, a third side proximate to a first side edge, and a fourth side proximate to a second side edge. an antenna housing for said antenna, said antenna housing having a shape for insertion within the slot in the computer housing. 19. The microstrip Antenna assembly of Claim 18 wherein said microstrip antenna is positioned within said antenna housing so that, when inserted in the slot in the computer housing, the first side edge is facing outward therefrom. 20. The microstrip antenna assembly of Claim 18 further comprising: a radio frequency (rf) front end positioned within said antenna housing; and a digital processing and interface circuit for interfacing radio frequency signal with said electronic equipment, said circuit positioned within said antenna housing. 21. The microstrip antenna assembly of Claim 18 wherein said antenna housing has a shape for a standard PCMCIA card, and includes connectors for connecting to standard PCMCIA connections inside the electronic equipment. 22. The microstrip antenna assembly of Claim 21 wherein said microstrip antenna is positioned within said antenna housing so that, when inserted in the slot in the computer housing, the first side edge is facing outward therefrom. 23. The microstrip antenna assembly of Claim 22 wherein said antenna housing further includes: a radio frequency (rf) front end positioned within said antenna housing; and a digital processing and interface circuit for interfacing radio frequency signal with said electronic equipment, said circuit positioned within said antenna housing. 24. A double ring microstrip antenna for use with a computer that is housed within a computer housing, said microstrip antenna comprising: a ground plane layer comprising a conductive material; a radiating patch layer comprising a conductive material; a dielectric layer positioned between the ground plane layer and the radiating patch layer; a shorting conductor coupling the radiating patch layer to the ground plane layer, said shorting conductor positioned on a shorted edge of the microstrip antenna; wherein said double ring microstrip antenna defines a radiating edge positioned opposite the shorted edge, a first side edge, and a second side edge; wherein said radiating patch layer includes a rectangular ring formed therein, said rectangular ring having four sides including a first side proximate to the radiating edge, a second side parallel to the shorting conductor, a third side proximate to a first side edge, and a fourth side proximate to a second side edge; and wherein said microstrip antenna is positioned within said computer housing. 25. The double ring microstrip antenna of Claim 24 wherein the shorting conductor couples a length between ten percent and ninety percent of the total length of the shorted edge. 26. The double ring microstrip antenna of Claim 24 wherein the shorting conductor couples a length between twenty percent and fifty percent of the total length of the shorted edge.

Inventors: SANAD MOHAMED S, SANAD, MOHAMED, S.
Applicants: WIRELESS ACCESS INC, WIRELESS ACCESS, INC.
Application year: 1994
Main classification: H01Q1
IPC codes: H01Q1, H01Q9, H01Q13